This invention relates to the fabrication of a strong composite plastic structure, and is particularly concerned with procedure for fabricating a high strength core sandwich structure in the form of a lightweight filler material or core positioned between high strength skin material or sheets, and with the resulting novel article, having particular utility as aircraft structural components.
In the production of aircraft components, it is conventional to employ a so-called "honeycomb core sandwich" structure, wherein a honeycomb formed of metal or plastic material, is positioned between and connected to outer skins of metal or fiber matrix material. However, such prior art structures are subject to certain disadvantages. In the first place, honeycomb core sandwich structures are relatively expensive to fabricate and when the honeycomb is formed of metal, presents corrosion problems. Further, it has been sought to provide components of reduced weight, yet having equivalent strength.
U.S. Pat. No. 4,013,810 to Long is directed to a sandwich panel construction comprised of a foam core of thermoplastic resin mixed with glass spheres sandwiched between two face sheets of fiber reinforced thermoplastic resin, wherein the thermoplastic resin of the core is the same as the thermoplastic resin of the face sheets. The apparently preferred thermoplastic resin employed according to the patent is polysulfone resin.
In Long, the sandwich panel thereof is produced by mixing a thermoplastic resin with a solvent to dissolve the resin, mixing hollow glass spheres with the dissolved thermoplastic resin, casting the mixture of the glass spheres and dissolved thermoplastic resin into a desired shape, drying the cast mixture to form a foam core, compression molding a mixture of thermoplastic resin with glass fibers to form a face sheet, and remolding under heat and pressure the face sheet to cause the thermoplastic resin to flow and to bond the face sheet to the foam core.
However, thermoplastics such as the polysulfone of Long, have the disadvantage that they soften by heating at relatively low temperatures, e.g. at temperatures ranging from about 160.degree. to about 250.degree. F., depending on the particular type of thermoplastic resin employed, thus limiting the upper temperature usage of the sandwich panels of the above patent.
Further, the thermoplastic sandwich structures of Long have the additional disadvantage that they are compression molded at high temperatures ranging from about 600.degree. to 700.degree. F. and utilizing pessures up to 200 psi.
It is also noted that Long states in the paragraph bridging colums 3 and 4 of the patent that if good adhesion between the core and the face sheets is required, it is advisable to interpose a layer of the thermoplastic resin common to the core and the face sheets, at the interface between the core and the respective face sheets before the face sheets are remolded to match the contour of the core.
U.S. Pat. No. 3,707,434 to Stayner dislcoses a rigidified resinous laminate or sandwich panel, comprising an outer sheet of an acrylic resin, a layer of polyester resin bonded to the acrylic sheet, such layer containing glass fibers and glass beads distributed therethrough, with a co-mingled second layer of a polyester resin bonded to the first layer, the second layer containing hollow plastic spheres in a stressed condition.
The method of making the reinforced plastic laminate of Stayner comprises applying the first polyester resin containing glass fibers and solid inert spheres onto an acrylic sheet, applying over the resulting uncured polyester surface a self-curing composition including a polyester resin, hollow plastic spheres and a curing agent for the polyester, and setting the polyesters by means of an exothermic reaction which expands the hollow plastic spheres, whereby upon cooling, the hollow plastic spheres are left in a stressed state.
However, the resinous laminate of Stayner is a relatively complex structure.
It is accordingly an object of the present invention to produce simple relatively inexpensive lightweight composite plastic structures particularly useful in the fabrication of aircraft structural components. A further object is the provision of procedure for production of a core sandwich structure comprised of a lightweight core material positioned between and connected to high strength outer skins, and to the sandwich structure so produced. A further object is to provide a core sandwich structure having the above characteristics, and which essentially avoids the above noted disadvantages of the prior art core sandwich structures.